Nitrogen oxides (NOx) in gases discharged from thermal power plants, various kinds of factories, automobiles and the like are substances causing photochemical smog and acid rain. As a nitrogen oxide removing technology, an exhaust gas denitrification process by selective catalytic reduction reaction using ammonia (NH3) as a reducing agent is known and is widely used mainly in thermal power plants and the like. Since the exhaust gas from the thermal power plants using coal and heavy oil as fuels contains a large amount of soot and dust, a denitrification catalyst becomes clogged, which is likely to incur an increase in pressure loss and a deterioration in denitrification efficiency over time. As a denitrification catalyst for exhaust gas having a high concentration of soot and dust, a parallel flow type catalyst is known. Examples of the parallel flow type catalyst include a catalyst in which plural sheets of plate-like catalyst elements are superimposed and a catalyst formed to have a honeycomb structure. Among these, the catalyst in which plural sheets of plate-like catalyst elements are superimposed has excellent mechanical strength compared to the catalyst formed to have a honeycomb structure since the plate-like catalyst elements are reinforced by a metal base or the like.
In order to improve the denitrification efficiency of the parallel flow type catalyst composed of plate-like catalyst elements, there is an attempt to provide protrusions or through-holes which impart turbulence to the flow of the exhaust gas in the plate-like catalyst elements. For example, PTL 1 discloses a unit plate-like catalyst in which plural weir-like protrusions and plural through-holes, which are complementary to the protrusions in form, are formed in a plate-like catalyst which supports a component having catalytic activity on the surface. In addition, PTL 2 discloses a structure in which plural sheets of projecting strip parts are alternately laminated at a right angle so as to impart turbulence to the flow of gas in a flow channel. Further, PTL 3 discloses that plural catalyst structures obtained by laminating plate-like catalyst elements are provided at predetermined intervals in a gas flowing direction and the direction of projecting strip parts of the plate-like catalyst elements in the catalyst structure is arranged to form a 90-degree angle with respect to the direction of projecting strip parts of the plate-like catalyst elements in the catalyst structure on a subsequent stage.